This invention relates to a process for recovering actinide values. More specifically this invention relates to a process for treating waste carbonate solutions which have been used to scrub actinides and other values and organic radiolytic and hydrolytic degradation products from neutral organo-phosphorous compounds which have been used as extractants in nuclear fuel reprocessing cycles in order to recover the actinide values.
In the Purex process for reprocessing of irradiated nuclear fuels, a neutral organo-phosphorous extractant such as tri-n butyl phosphate (TBP) or dihexyl-N,N-diethyl carbamylmethylene phosphonate (DHDECMP) phosphate is diluted with a normal paraffin hydrocarbon such as n-dodecane and is used to recover the actinides from an aqueous nitric acid feed solution. During reprocessing the extractants are subjected to high radioactivity and other adverse conditions which cause the extractant to undergo significant radiolytic and hydrolytic degradation, forming acidic extractants such as dibutyl phosphoric acid (HDBP) and monobutylphosphoric acid (H.sub.2 MBP), which interfere with the extraction process and with the recovery of the actinides from the extractant. These degradation products are themselves good extractants, especially for the hexa- and tetravalent actinides and certain fission products such as ruthenium and zirconium. The extractants have especially high distribution ratios at the low acid concentrations used to back-extract U(VI) and Pu(IV) from TBP and U(VI), Np(IV), Pu(IV) and Am(III) from DHDECMP. Thus, extractable complexes of actinides and zirconium are formed during stripping operations. At high concentrations of degradation products, the extractable complexes may precipitate, sometimes in sufficient quantities to interfere with liquid-liquid extraction operations.
The TBP and DHDECMP extractant solutions must be cleaned up periodically so that they can be recycled back into the extraction process. This is presently done by contacting them with a sodium carbonate scrub solution which forms water-soluble sodium salts with the acidic extractants. In addition, the scrub solution also forms carbonate-complexes of the actinides and zirconium, which are fairly soluble in excess carbonate. Thus, the sodium carbonate scrub waste solution, resulting from contact with TBP-dodecane extractant, will consist essentially of NaHCO.sub.3 --NaNO.sub.3 and contain varying amounts of sodium dibutyl phosphate (NaDBP), disodium monobutyl phosphate (Na.sub.2 MBP), carbonato-actinide and carbonato-zirconium complexes, depending upon the extent of hydrolysis and radiolysis. Analogous wastes from DHDECMP-diisopropyl benzene (DIPB) processing are similar in composition but contain mono- and di-acidic salts formed by hydrolysis of the ester groups in DHDECMP.
At present, the scrub waste solutions containing the actinides and degradation products which result from the cleanup of the organic extractants are stored in stainless steel containers. However, since as much as 6 kg of actinide may be present in the sodium carbonate scrub waste solutions resulting from the clean up of extractants used to process one metric ton of light water reactor fuel, it is imperative that some method be found to recover and concentrate these actinides for appropriate and suitable long-term storage.
The efficient removal of actinides and fission products from the sodium carbonate scrub waste solutions presents several problems. For example, neutralization of the carbonate solutions with HNO.sub.3 followed by cation exchange results in poor metal ion absorption on the resin, precipitate formation of some of the metal ions as complexes of the degradation products, and in extensive column plugging. Acidification of the carbonate solutions with excess HNO.sub.3 followed by extraction with TBP or preferably DHDECMP, results in the rapid buildup of high concentrations of acid degradation products which prevent efficient back-extraction. Thus, there are no satisfactory methods for the efficient and effective recovery of the actinide values from the scrub waste solutions so that the actinides may be utilized or prepared for long-term storage.